Sachiko is a kind and caring woman who deeply loves her family. She worries about her husband Soichiro's well-being while he works on the Kira case. When Soichiro is hospitalized due to a stress-related heart attack, she is frequently by his side to care for him. As concerned as she is about Soichiro, she also understands his convictions in working on the Kira case, and she stays by his side. Sachiko is proud of the academic success of her son, Light, and congratulates him on his good grades. When her daughter Sayu is wheelchair-ridden due to emotional trauma caused by her kidnapping, Sachiko aides her recovery, encouraging her and taking her for walks outside.

Sachiko is first seen waiting for Light to show her his exam grades. She is often at home when Light returns from school and is seen when the family eats or relaxes together. After the timeskip, she denies support for a match between her daughter Sayu and Touta Matsuda.

Sachiko Yagami is apparently never told that her son Light is Kira. It is likely that she is told he died in the battle against Kira like his father Soichiro. Near makes sure that the truth of Kira's defeat is never publicized, and in the manga, people simply believe Kira is resting.

Sachiko's role in the film series is essentially the same as her role in the manga.

After Light is exposed as Kira and is killed by Ryuk, Soichiro never reveals the truth behind their son's death. He tells Sachiko and their daughter Sayu that Light was killed by Kira instead. The year after Light's death, at the end of the second film, Sachiko and Sayu make Light a birthday cake.

Sachiko dies from a terminal illness prior to the events of television drama, but she is shown in several flashbacks throughout the series. Her death strains Light's relationship with Soichiro due to the latter being unable to be present at her deathbed.

In one flashback, Light fondly remembers pretending to be a police officer while his mother happily watched him and his sister play.

Light's mother is unnamed in Netflix's 2017 Death Note film. Similar to the drama, Light's mother passed away before the events of the film. She was killed when Antony Skomal hit her with his car. In an act of revenge, Light kills Skomal, making him Light's second Death Note victim.

Light's father James Turner describes her as being sort of a hippy and that she said: "karma's a bitch."

James later talks to Light and Mia Sutton about her as well, saying that she wouldn't have been caught within ten miles of him in high school, but their conversation is interrupted by L.

Takeshi Obata said that he had no real concept behind the design of Sachiko; he claimed that he encountered difficulty in drawing female characters who were not the main characters. Tsugumi Ohba did not include Sachiko in the final chapter of the manga but stated that someone in the Task Force would likely tell her that Light and Soichiro died in order to stop Kira, as the Task Force members would never let the world know the truth about the Death Note and the Kira case.